"Coherence inferences are necessary to form a consistent and intelligible mental picture of a text. " (Coherence inferences. Retrieved from nationalstrategies.standards.dcsf.gov.uk)
Coherence inferences are those inferences that a reader needs to make to understand the text. Authors make assumptions that the readers have the necessary prior knowledge needed to fill-in the missing information. For example to be able to fully understand the following, "She saw smoke pouring out of the neighbor's house, and immediately dialed 911." You must make two coherence inferences. 1) The smoke pouring out of the house is not a good thing. It likely means there is a fire. 2) You call 911 when there is an emergency to get help. The author makes the reasonable assumption that the audience will have the necessary prior knowledge to make these inferences to maintain coherence.